Optical phase detector

ABSTRACT

An optical phase detector comprising coupling means for receiving two optical inputs and for producing two combined optical outputs, means for detecting the two combined optical outputs and producing two corresponding electrical signals, and means for measuring the difference between the two electrical signals and generating an output difference signal which may be used to provide an indication of the phase difference between the two optical inputs. The optical phase detector comprises a voltage-tuneable electro-optic phase modulator for modulating the phase of an optical input to the optical phase detector to provide a linearized response. In this arrangement the output difference signal may be maintained at a constant level by varying the voltage applied to the electro-optic phase modulator, the applied voltage providing an indication of the phase difference between the two optical inputs. Applications include frequency discriminators, various sensors, and a laser stabilization apparatus.

This application is the U.S. national phase of international applicationPCT/GB99/03179, filed in English on 22 Sep. 1999 which designated theU.S. PCT/GB99/03179 claims priority to GB Application No. 9820493.6filed 22 Sep. 1998. The entire contents of these applications areincorporated herein by reference.

The invention relates to an optical phase detector for measuring thephase difference between two input signals. The invention also relatesto applications of an optical phase detector, including use in anoptical frequency discriminator and a laser stabilisation apparatus.

By way of background to the present invention, U.S. Pat. No. 5,396,166describes a fibre optic interferometer sensor system including a fibreoptic interferometer having an electrostrictive transducer bonded to oneof the first and second optical fibre arms of the interferometer. Theelectrostrictive transducer has non-linear characterstics which enablesdetection of low frequency or DC signals at side bands of a highfrequency carrier. Also by way of background to the invention, EP0388929describes a fibre optic laser gyro in which a modulated phase differenceis detected between two rays of light phase modulated by means of aphase modulator having propagated through an optical fibre coil inopposite directions and having interfered with each other.

Conventionally, phase difference may be measured optically byinterfering two beams to form an interference fringe pattern andmeasuring the fringe pattern as it moves across a camera face due to achange in the relative phase. A disadvantage of this type of measurementis that it relies on the counting of fringes and interpolation betweenfringes to measure phase or position accurately. This process can berelatively slow and not especially accurate. Such detectors may be usedin displacement measurement schemes, such as in laser metrology, tomeasure position accurately i.e. to a fraction of an optical wavelength.

Optical techniques have previously been used for the generation ofmicrowave radiation, by mixing two stable lasers, and for phased arrayantenna beam-forming, in which simple optical systems are used toperform a complicated microwave function. A known technique for thegeneration of stabilised microwave radiation has been achieved byincorporating a fibre optic delay line in an RF, microwave or millimeterwave (mm-wave) frequency discriminator circuit. This enables thedifference in frequency of two stable laser inputs to be controlledaccurately and therefore the RF, microwave or mm-wave output can bestabilised (British Patent Application No. GB2307332). In this systemthe frequencies of the lasers can drift but the difference in frequencybetween them remains constant. It is an object of the system to providean apparatus for providing stable radiation at RF frequencies.

In many applications high spectral purity (i.e. stable) laser radiationis required. These applications include fundamental physics research,for example graviton detection, photochemistry, luminescence excitationspectroscopy, absorption and Raman spectroscopy, and applications suchas fibre optic communications, sensors, laser radar, laser air speedindicators and laser vibrometry. However, lasers of well definedfrequency (or wavelength) and high spectral purity (e.g. narrowlinewidth) tend to be expensive and complex. High spectral purity isattainable with certain gas lasers, but there is a need for solid statelasers of similar or superior performance. Solid state lasers includelaser diodes and diode pumped YAG lasers. The most widely used laserdevice is the laser diode. Although these devices are relatively cheap,devices of this kind have a particularly poor spectral stability,especially in the case of Fabry-Perot etalon designs which often supportseveral modes simultaneously.

For many applications, it is useful to overcome the problem of poorspectral quality and to be able to stabilise the frequency output of asingle laser. Furthermore, for some applications, extremely pure laserradiation is required.

The stabilisation of a laser output with an external component haspreviously been reported [FM noise reduction and sub kilohertz linewidthof an AlGaAs laser by negative electrical feedback, M. Ohutso et al.,IEEE Journal of Quantum Electronics 26 (1990) pp 231-241]. In thissystem, the external component is one or more high finesse Fabry-Perot(FP) interferometer. Stabilisation of the laser is achieved by utilisingthe reflectance characteristics of the interferometer or interferometersto detect FM noise. Electrical feedback is then ised to feed back thissignal to correct the laser output. However, the system is a complex,free-space system which gives cost disadvantages. Also, because thesystem operates in free space it is particularly susceptible to externalactors such as vibration, air circulation and dust and also to changesin temperature.

By way of background to the present invention, U.S. Pat. No. 4,972,424describes a laser stabilisation apparatus for stabilising a lasercavity. The apparatus employs a piezoelectric transducer controlling theposition of one of the cavity mirrors, the transducer being driven tochange the effective cavity length in response to measurement of thelaser output power.

It is an object of the present invention to provide a laserstabilisation apparatus which overcomes these problems. It is a furtherobject of the invention to provide an optical phase detector which maybe included in the laser stabilisation apparatus.

According to a first aspect of the present invention, an optical phasedetector comprises;

-   means for receiving two optical inputs and producing two combined    optical outputs,-   detection means for detecting the two optical outputs and converting    the intensity of each of the combined optical outputs into an    electrical signal,-   means for measuring the difference between the two electrical    signals and generating an output difference signal, and-   a voltage-controlled electro-optic phase modulator for modulating    the phase of one optical input to the optical phase detector, the    electro-optic phase modulator having a substantially linear response    whereby, in use, the output difference signal is maintained at a    substantially constant level by varying the voltage applied to the    electro-optic phase modulator, the voltage being applied to the    modulator by means of a feedback loop in response to the output    difference signal, the applied voltage providing an indication of    the phase difference between the two optical inputs.

The optical phase detector may include coupling means for receiving twooptical inputs and producing two combined optical outputs. The couplingmeans may be any means for producing two combined optical outputs fromthe two optical inputs, wherein the coupling means produce twointermediate optical outputs from each of the optical inputs, the twointermediate optical outputs produced from each of the optical inputsbeing in phase quadrature, and wherein the intermediate optical outputsare combined to form the two combined optical outputs. For example, thecoupler may be an optical fibre coupler, or other coupled waveguidedevice, such as in integrated optic waveguide coupler.

Preferably, the constant level is substantially zero volts. As thevoltage applied to the elect-optic phase modulator provides anindication of the phase difference between the two optical inputs, thisprovides the advantage that the optical phase detector is linearised asthe voltage required to drive the electro-optic phase modulator isdirectly proportional to the phase difference between the two opticalinputs. The voltage is applied to the electro-optic modulator by meansof a feedback loop from the optical phase detector output. The opticalphase detector may therefore further comprise means for feeding back theoutput difference signal to the electro-optic phase modulator, theapplied voltage to the electro-optic modulator being varied in responseto the output difference signal so as to maintain the difference signalat the substantially constant level.

The optical phase detector provides the advantage that it provides anelectrical output signal indicative of relative phase difference betweenthe two optical input signals. This cannot be achieved with conventionalelectrical phase detectors due to the high optical frequencies of theinputs.

Furthermore, although phase difference may be measured optically byinterfering two beams to form an interference fringe pattern andmeasuring the fringe pattern as it moves across a camera face due to achange in the relative phase, this relies on the counting of fringes andinterpolation between fringes to measure phase or position accurately.The optical phase detector of the invention is advantageous as theoutput occurs as a direct electrical signal giving a linear measure ofphase difference over an extended range which may be at least 360°.

The electro-optic phase modulator may comprise an optical waveguide onan integrated optic substrate, the substrate preferably being any oflithium niobate, lithium tantalate or gallium arsenide. The frequencyresponse of the electro-optic phase modulator may be at least 1 MHz and,preferably, may be at least 1 GHz. Alternatively, the electro-opticphase modulator may take the form of an optical fibre carrying apiezoelectric material. For example, the piezoelectric material may bedeposited on, or otherwise attached to, the optical fibre. The opticalfibre may be stripped of its outer cladding.

Preferably, the two optical inputs to the coupling means havesubstantially equal amplitudes. The two optical inputs may be derivedfrom the same source of radiation, for example a single laser. In thisarrangement, the optical phase detector provides an electrical outputdependent on the phase difference between two optical inputs from thesame source of radiation, therefore providing a measure of the relativephase difference. Alternatively, in some applications, the two opticalinputs may be derived from two different sources of radiation,preferably having the same amplitude.

The optical phase detector may also comprise polarisation modulationmeans for modulating the polarisation of at least one of the inputs tothe optical phase detector so as to ensure the polarisation of the twoinputs is substantially the same. Typically, the polarisation modulationmeans may be a fibre-optic or integrated optic polarisation modulator.

The optical phase detector may comprise two photodetectors, each one fordetecting the intensity of one of the optical outputs and for generatingan electrical output signal in response to the corresponding opticaloutput. Preferably, the photodetectors are matched photodetectors.

According to a second aspect of the invention, a frequency discriminatorapparatus comprises;

-   an optical phase detector as described herein,-   means for receiving a primary optical input from a source of    radiation and for producing two primary optical outputs, and-   means for introducing a relative delay between the two primary    optical outputs,-   the two primary optical outputs, having a relative delay    therebetween, providing the inputs to the optical phase detector.

For example, the frequency discriminator may include primary couplingmeans for producing two primary optical outputs from the primary opticalinput. Preferably, the two primary optical outputs have substantiallythe same amplitude.

In the frequency discriminator apparatus, the electrical output from theoptical phase detector provides a measure of the optical frequency (orwavelength) of an input laser providing the relative delay is known andis substantially stable.

Preferably, the means for introducing a relative delay between the twoprimary optical outputs comprise two lengths of optical fibre havingdifferent optical path lengths. For example, one length of optical fibremay be used through which one of the primary optical outputs istransmitted, with the other primary optical output being transmittedthrough a negligible length of optical fibre. Alternatively, anothertype of delay medium may be used, for example an integrated optic de ayline.

The one or more length of optical fibre may be single mode opticalfibre, temperature stable, single mode optical fibre or temperaturestable polarisation maintaining optical fibre. The use oftemperature-stable optical fibre provides the advantage that theapparatus has improved temperature stability. Alternatively, or inaddition, to achieve temperature stability, the apparatus may be housedin a temperature-stable oven.

The frequency discriminator apparatus, comprising an optical phasedetector as herein described, provides a sensor for measuring any oneof, for example, temperature, pressure or strain, when that measurand isapplied to the optical fibre delay line. It may be advantageous in asensor application if the relative optical delay in the apparatus issubstantially zero.

According to another aspect of the present invention, a laserstabilisation apparatus for stabilising the output from a source ofradiation comprises;

-   a frequency discriminator apparatus comprising input means for    receiving a primary optical input from a source of radiation having    a frequency, and for producing two primary optical outputs,-   means for introducing a relative delay between the two primary    optical outputs, the two primary optical outputs, having a relative    delay therebetween, being input to an optical phase detector,    wherein the optical phase detector comprises coupling means for    receiving the two optical inputs and producing two combined optical    outputs, detection means for detecting the intensity of the two    combined optical outputs and converting the intensity of each of the    combined optical outputs into an electrical signal, and means for    measuring the difference between the two electrical signals and    generating an output difference signals,-   the laser stabilisation apparatus further comprising feedback means    for feeding back the output difference signal from the optical phase    detector of the frequency discriminator to the source of radiation.

The source of radiation may be a laser, the laser having a suitabletuning point such that the output difference signal may be fed back tothe tuning point.

The input means for receiving the primary optical input may be inputcoupling means, such as a coupler, or a beam splitter.

The apparatus is less complex than the known apparatus and is thereforecheaper to construct. It also avoids the problems of free space optics.Also, the laser stabilisation apparatus is capable of stabilising theoutput from a laser over a narrow to broad frequency range which may bevaried depending on requirements, for example through the choice of thedifferential delay time.

In one embodiment of the laser stabilisation apparatus, the apparatusmay comprise one or more additional frequency discriminator, eachfrequency discriminator having corresponding feedback means for feedingback the electrical output from the associated optical phase detector tothe source of radiation. The outputs from the optical phase detectors ofthe different frequency discriminators may be fed back to differentcontrol points on the source of radiation.

The apparatus also enables temperature stability, through the use oftemperature-stable optical fibre or other delay means, as well as shortterm stability to be imparted to the output laser spectrum. Theapparatus is also relatively insensitive to vibration and dust.

The optical phase detector forming part of the laser stabilisationapparatus may include a voltage-controlled electro-optic phase modulatorfor modulating the phase of one optical input to the optical phasedetector, the electro-optic phase modulator having a substantiallylinear response.

The invention also relates to a method of stabilising the output from alaser, using the laser stabilisation apparatus as herein described.

The optical phase detector of the invention may also be used in otherapplications, such as laser metrology, in a displacement measurementscheme capable of measuring nanometre displacements. This aspect of theinvention provides an advantage over known displacement measurementschemes which rely on the counting of fringes and interpolation betweenfringes to measure position accurately.

According to another aspect of the invention, an optical frequencysynthesizer comprises;

-   the laser stabilisation apparatus as herein described, and-   means for varying the frequency of the laser output.

The laser stabilisation apparatus forming part of the optical frequencysynthesizer may include two lengths of optical fibre through which theprimary optical outputs are transmitted, the two optical fibres havingdifferent optical path lengths.

Preferably, the optical frequency synthesizer may include anelectro-optic phase modulator arranged in the path of one of the lengthsof optical fibres, whereby application of a SAWTOOTH-like voltagewaveform to the electro-optic phase modulator gives rise to a variationof the frequency of the laser output. Typically the electro-optic phasemodulator may be arranged in series with one of the lengths of opticalfibres.

The optical frequency synthesizer may also comprise a voltage source,providing an SAWTOOTH-like voltage waveform, for applying a voltage tothe electro-optic phase modulator.

Alternatively, the optical frequency synthesizer may comprise adifferential amplifier, the output from the optical phase detector beingfed back to an input of the differential amplifier, the output from thedifferential amplifier being fed back to the laser.

The optical phase detector included in the laser stabilisation apparatusforming part of the optical frequency synthesizer may preferablyinclude, but need not include, an electro-optic phase modulator.

According to another aspect of the invention, an optical vectorvoltmeter for comparing an input laser signal and a reference signalcomprises;

-   the optical phase detector as herein described,-   a photodetector for receiving the input laser signal and for    generating an output signal dependent on the amplitude of the input    laser signal,-   the output from the electro-optic phase modulator providing a    measure of the phase difference between the reference signal and the    input laser signal.

Preferably, the optical phase detector included in the optical vectorvoltmeter includes an electro-optic phase modulator. This provides theadvantage that there is a linear output covering at least 360°.

According to another aspect of the invention, an optical networkanalyser for measuring the transmitted or reflected amplitude and phaseof a system at a plurality of frequencies comprises;

-   an optical frequency synthesizer for generating a reference signal    at a plurality of frequencies, and-   the optical vector voltmeter as herein described, for receiving as    inputs the reference signal and the signal transmitted or reflected    by the system.

The optical frequency synthesizer included in the optical networkanalyser may be of the type herein described, or may be a conventionaloptical frequency synthesizer.

In any of aspects of the invention described above, single mode opticalfibre, polarisation maintaining optical fibre, temperature stable singlemode optical fibre or temperature stable polarisation maintainingoptical fibre may be used.

Although aspects of the invention are herein described as in-fibreapplications, all aspects of the invention may make use of free spaceoptics in all or some of the optical paths. For example, free spaceoptics may be used in a gas sensor. Similarly, integrated optics may beemployed.

The invention will now be described, by way of example only, withreference to the following figures in which;

FIG. 1 shows a diagram of a conventional optical phase detector,

FIG. 2 illustrates the output levels of the detectors of the opticalphase detector of FIG. 1 as a function of relative optical phase,

FIG. 3 illustrates the differential output level between the detectoroutputs shown in FIG. 2 as a function of relative optical phase,

FIG. 4 shows a diagram of a linearised version of the optical phasedetector shown in FIG. 1,

FIG. 5 shows the linearised optical phase detector shown in FIG. 4including an electrical feedback loop,

FIGS. 6(a) and 6(b) shows experimental results obtained using thelinearised optical phase detector, with feedback, as shown in FIG. 5,

FIG. 7 shows an example of a practical electrical circuit of thedetectors included in the optical phase detector in FIG. 1,

FIG. 8 shows the laser stabilisation apparatus of the invention,including the optical phase detector shown in FIG. 1,

FIG. 9 shows phase noise measurements for unstabilised laser outputs andstabilised laser outputs obtained using the laser stabilisationapparatus of the present invention, for (a) a Lightwave ElectronicsSeries 123 Fibre-coupled Diode-pumped solid state non-planar ring laseroperating at 1319 nm and (b) an E-Tek DFB laser, type LDPM, operating at1550 nm,

FIG. 10 shows the laser stabilisation apparatus shown in FIG. 8, andfurther comprising an electro-optic phase modulator, or a differentialamplifier in the feedback loop of the apparatus,

FIG. 11(a) shows an example of a SAWTOOTH voltage waveform which may beapplied to the phase modulator in the apparatus of FIG. 10 to provide anoptical frequency synthesizer, and FIG. 11(b) shows the variation ofphase of the electro-optic phase modulator to which a SAWTOOTH waveformvoltage is applied,

FIG. 12 shows an experimental result obtained using the apparatus shownin FIG. 10,

FIG. 13 shows a schematic diagram to illustrate how the optical phasedetector shown in FIG. 5 may be employed in a sensor,

FIG. 14 shows a schematic diagram to illustrate how the optical phasedetector shown in FIG. 5 may be employed in an optical vector voltmeter,

FIG. 15 shows a schematic diagram to illustrate how the optical vectorvoltmeter shown in FIG. 14 and the apparatus shown in FIG. 10 may beemployed in an optical network analyser for transmission measurement ofa system under test,

FIG. 16 shows a schematic diagram to illustrate how the optical vectorvoltmeter shown in FIG. 14 and the apparatus shown in FIG. 10 may beemployed in an optical network analyser for reflection measurement of asystem under test, and

FIG. 17 shows a schematic diagram of a gas sensor application of theoptical phase detector and employing free space optics.

It is an object of one aspect of the invention to provide an opticalphase detector capable of producing an electrical output signaldependent on the phase difference, or relative phase, between twooptical inputs of substantially the same frequency.

Referring to FIG. 1, an optical phase detector, referred to generally as1, comprises a 50/50 fibre optic directional coupler 2 for coupling twoinputs 3, 4. The detector 1 also comprises two optical detectors 5 a, 5b and a differential amplifier 6. In operation, the two inputs 3, 4 areinput to the optical phase detector 1 through the directional coupler 2.A 50/50 single mode fibre optic directional coupler has a centralportion 10 comprising two coupled optical fibres.

The inputs 3, 4 to the coupler are combined by evanescent coupling inthis central portion 10 and two combined outputs 11, 12 are generated.These combined outputs 11, 12 will vary in amplitude (and intensity)depending on the phase difference between the two input signals 3, 4.Preferably, the input signals 3, 4 have substantially equal amplitudes.

The function of the coupler is to produce two intermediate outputs fromeach of the two inputs (i.e. four in total) which are then combined toform the two outputs 11, 12. Each input signal gives rise to twointermediate optical outputs and it is an important property of thecoupler that the two intermediate outputs produced from each of theindividual inputs are in phase quadrature (i.e. have a phase differenceof substantially 90° therebetween). The intermediate outputs from eachof the inputs are combined in pairs to provide the combined outputs 11,12. The coupler may be any coupling means which provides this function.Ideally, this coupler may be a coupled waveguide device, such as aconventional in-fibre coupler, but other coupler devices may be used.

Typically, the two optical inputs 3, 4 to the optical phase detector maybe derived from the same source, such as a laser (not shown). It ispreferable for the optical inputs 3, 4 to be taken from the laser outputto the phase detector via single mode optical fibre. Alternatively, twobeams of radiation may be taken from the laser and coupled to singlemode optical fibres, for example by means of lenses, for subsequentinput to the optical phase detector 1.

Each of the output signals 11, 12 from the coupler 2 is directed to aseparate detector 5 a, 5 b which converts the intensity of therespective optical input (i.e. inputs 11, 12) into a correspondingelectrical output signal 7 a, 7 b. The electrical signals 7 a, 7 boutput from the detectors 5 a, 5 b are then passed to a differentialamplifier 6 which produces an output 20 proportional to the differencein voltage between the input signals it receives. Preferably, thedetectors are matched (i.e. are as identical as possible). For example,they may be made in the same batch or even on the same chip. Forclarity, the electrical connections to the detectors are not shown inFIG. 1.

The optical phase detector 1 exploits the fact that the two outputsignals 7 a, 7 b from the detectors 5 a, 5 b depend on the relativephase of the two optical signals 3, 4 input to the coupler 2. That is,the detectors 5 a, 5 b and the differential amplifier 6 provide asubstantially balanced optical detector circuit (referred to generallyas 32) and the voltage output from the differential amplifier 6 gives ameasure of the phase difference between the two input signals 3, 4. Thisrequires the amplitudes of the inputs 3, 4 to be substantially constant.If the amplitudes of the inputs 3, 4 are varying, it is possible tomeasure the amplitudes separately and correct for any variationelectrically.

In order to measure the phase difference between two optical inputs 3, 4it is important to ensure the polarisation of the two inputs 3, 4 is thesame. This may be achieved by including a polarisation controller in thepath of one or both of the input signals 3, 4. For example, if the inputsignals 3, 4 are coupled to the phase detector 1 from a laser by meansof optical fibre a fibre optic polarisation controller may be used inone, or both, of the input pathways 3, 4. Alternatively, in anintegrated optical phase detector, an integrated optic polarisationmodulator may be used. Polarisation-maintaining optical fibres andcouplers may be used throughout the optical phase detector 1 also.

This behaviour of the optical phase detector 1 is illustrated in FIGS. 2and 3. FIG. 2 shows how the outputs 7 a, 7 b from the individualdetectors vary with the relative phase between the two optical inputs 3,4. The amplitudes of the output signals 7 a, 7 b from the two detectors5 a, 5 b vary with relative phase as a squared-sinusoid and arerelatively in anti-phase. The optical phase detector 1 thereforeprovides a DC output voltage which is a measure of the phase differencebetween the two input signals 3, 4.

FIG. 3 shows the differential output 20 of the pair of detector outputs7 a, 7 b which varies sinusoidally with the relative optical phase ofthe inputs 3, 4. The output 20 is bipolar and is particularly useful insome applications, as will be described in more detail later. However,in accordance with a first aspect of the present invention, an improvedoptical phase detector includes a voltage controlled electro-optic phasemodulator to provide an optical phase detector having a linearisedresponse. The “voltage-controlled” electro-optic phase modulator mayalso be referred to as a “voltage-tuneable” electro-optic phasemodulator.

Typically, an electro-optic phase modulator comprises an opticalwaveguide on an integrated optic substrate, such as lithium niobate,lithium tantalate, gallium arsenide or other electro-optic material. Byapplying a voltage across the waveguide by means of metallic electrodesthe phase of radiation passing through the waveguide can be modulated.The response of electro-optic phase modulators comprising a lithiumniobate or lithium tantalate substrate is very linear. In addition, thefrequency response of such electro-optic phase modulators is very fast,typically up to 1 GHz or higher.

FIG. 4 shows a manually operated optical phase detector 30 in accordancewith one aspect of the present invention. The optical phase detector 30has a linearised response, and comprises a voltage controlledelectro-optic phase modulator 35 in one of the input paths 3, 4 (input 3in FIG. 4). The electro-optic phase modulator 35 has a substantiallylinear variation of optical phase with applied voltage 36. If thisvoltage were manually (or automatically) adjusted to maintain a constant(ideally zero) output 20 from the differential amplifier the voltage 36applied to the modulator 35 gives a direct linear measure of therelative phase between the optical inputs 3, 4. In practice, automatic,as opposed to manual, adjustment of the applied voltage 36 would behighly desirable. The range of electro-optic phase modulation may be atleast 360°.

The electro-optic phase modulator 35 may be placed in the path of one ofthe inputs 3, 4 to the optical phase detector 1. The output 20 from thedifferential amplifier 6 is then set at a constant level by applying theappropriate voltage to the voltage-tuned electro-optic phase modulator.Preferably, the output 20 may be maintained at zero volts which makesthe measurement insensitive to amplitude variations.

As the response of electro-optic phase modulators is very linear, inparticular lithium niobate and lithium tantlate devices, the voltagerequired to set the differential amplifier output to zero gives a usefulmeasure of the relative phase. The frequency response of theelectro-optic phase modulator may, typically, be between 1 MHz and 1GHz. For some applications, it may be desirable to include anelectro-optic phase modulator having a frequency response exceeding 1GHz. The voltage adjustment may be made manually, or by means of anelectrical feedback loop. Including the electro-optic phase modulatorprovides the advantage that the phase detector is linearised (i.e. thevoltage required to tune the electro-optic modulator in order tomaintain the differential amplifier at a constant voltage is directlyproportional to the measured phase difference) and the operating rangeis extended to at least 360°. If the difference signal is maintainedclose to zero the optical phase detector has the further advantage thatit is insensitive to amplitude variations in the input optical signals.Additionally, as the frequency response of the electro-optic modulator35 is very fast, the phase measurement can be determined rapidly. Thephase detector is therefore suitable for dynamic measurements. Inpractice, the speed of response of the optical phase detector may belimited by the detector or feedback electronics.

In an alternative embodiment, the electro-optic phase modulator may takethe form of an optical fibre, on which a piezoelectric film isdeposited, for example ZnO, the optical fibre being stripped of itsouter cladding. By applying a voltage across the ZnO film on the fibre,a phase shift can be induced. This embodiment may be advantageous as itavoids the size, cost and insertion loss of an integrated optic phaseshifter.

FIG. 5 shows an embodiment of the linearised optical phase detector 30which includes an electronic feedback loop, comprising a feedbackamplifier 44 and filter 46. As shown in the figure, the feedbackamplifier 44 and filter 46 may be separate components, but alternativelythe filter may be derived from the amplifier characteristic itself whichis carefully designed to be stable. The voltage 36 applied to theelectro-optic phase modulator 35 provides the output, which islinearised. The advantage of this embodiment of the phase detector isthat it removes the need for a human operator and is relatively fast andaccurate in operation. FIGS. 6(a) and 6(b) show experimental resultsobtained using the device shown in FIG. 5. The measurements wereobtained directly from a Tektronix 2430 digital oscilloscope. The solidlines are the best fits to the measured points. This characteristic maybe compared with the non-linear (sinusoidal) response of FIG. 3.

The optical phase detector shown in FIG. 5 also provides an advantageover that shown in FIG. 1 as it has an extended operating range. In theoptical phase detector shown in FIG. 1, the output is nonlinear(sinusoidal) and the greatest unambiguous operating range is therefore180°. The optical phase detector of the present invention has a verylinear response and the range can be extended to at least 360°, as shownin FIG. 6(b). It is particularly important, for example, to have anoperating range greater than 360° in vector voltmeter or networkanalyser applications, as will be described hereinafter.

A circuit diagram of part of the optical phase detector 30 is shown inFIG. 7. In a non-linear optical phase detector 1 (i.e. as shown in FIG.1), the detectors 5 a, 5 b are be arranged such that the net outputsignal 20 varies sinusoidally about zero depending on the relative phasebetween the inputs 11, 12. Hence a bipolar signal 20 is output from thedetector (as shown in FIG. 3). Typically, the detectors 5 a, 5 b may bereverse biased PIN devices, although any low noise detector may be used.Commercially available optical detectors GAP 60 or GAP 100 may besuitable devices. The value of resistor R₁ (as shown in FIG. 7)determines the gain and hence the peak to peak bipolar output voltage.Typically, the voltage (+V, −V) applied across the detectors may be ±9Volts.

If the photodetectors 5 a, 5 b are not quite identical, means forequalising the sensitivity of the photodetectors may be included in thephase detector. It has been found that the bias voltage on one or bothdetectors may be varied in order to provide a fine adjustment tophotodetector sensitivity.

The linearised optical phase detector 30 provides a DC output voltagewhich is a measure of the phase difference between the two inputs. Thiscannot be achieved with conventional electrical phase detectors due tothe high optical frequencies of the inputs. Furthermore, the opticalphase detector of the invention is advantageous as the output occurs asa direct electrical signal giving a measure of phase difference. Thedetector may therefore be used in a high accuracy displacementmeasurement apparatus for measuring displacements of the order of a fewnanometres. This may be of particular use in laser metrology. Forexample, the output from the optical phase detector provides a measureof the phase difference between two input signals derived from the same,stable laser, where one of input signals is reflected from a displacedsurface or object. The phase difference may be used to provide anindication of the displacement of the surface or object. The input lasermay be stabilised by the laser stabilisation apparatus of the inventionwhich will be described later.

The optical phase detector may also be used to phase lock twoindependent lasers, for example a high powered tuneable laser and a lowpower stable laser. This is achieved by inputting laser outputs from twodifferent lasers to the optical phase detector (i.e. as inputs 3 and 4shown in FIGS. 1 and 4) and employing a feedback loop connected to thetuning point of the tuneable laser.

The characteristics of the optical phase detector of the presentinvention may also be exploited in a frequency discriminator. In generalterms, a frequency discriminator employs an input signal of ideallyconstant amplitude and produces an output voltage proportional to, ordependent on, the amount by which the input frequency differs from afixed frequency.

Referring to FIG. 8, there is shown a laser stabilisation apparatus inaccordance with another aspect of the invention, as will be describedhereinafter. The ports of the apparatus enclosed within the dotted line,referred to generally as 60, form a frequency discriminator comprisingan optical phase detector 1 (as shown in FIG. 1) and a 50/50 fibre opticdirectional coupler 41 for receiving an output 42 from a laser 43. Thediscriminator 60 also comprises means for introducing a relative delaybetween the two output signals 52, 53 from the coupler 41. Preferably,this may be achieved by means of two lengths of optical fibre 50, 51,each for receiving one of the output signals 52, 53 from the coupler 41,one optical fibre being longer than the other so as to introduce arelative delay. In practice it is convenient if one of the lengths ofoptical fibre is very short. Preferably, the one or more length ofoptical fibre 50, 51 may be temperature-stable optical fibre which givesthe apparatus improved temperature stability. Alternatively, or inaddition, to achieve temperature stability, the apparatus may be housedin a temperature-stable oven. A polarisation modulator, such as anin-fibre polarisation modulator, may be included in either one or bothpathways 50, 51, as discussed previously.

It will be appreciated that, although the electro-optic modulator 35within the optical phase detector is not shown in the frequencydiscriminator 60 forming part of the laser stabilisation apparatus inFIG. 8, the electro-optic modulator 35 may be included, and may beparticularly important in other applications of the discriminator.

The function of the frequency discriminator 60 is to split the output ofa laser 43 into two substantially equal output beams 52, 53 by means ofthe first coupler 41, delay one signal relative to another and thenmeasure the optical phase difference between the two signals 52, 53.This may be done by means of the optical phase detector of the presentinvention. It cannot be done using conventional electrical phasedetectors as the optical frequencies in question are far too high.

One output 53 from the first coupler 41 is connected through an opticalfibre 51 to a second 50/50 fibre optic coupler 2, which forms part ofthe optical phase detector 1. The other output 52 is passed through thelonger optical fibre (i.e. a delay line 50) before connection to thiscoupler 2. Alternatively, another type of delay medium may be used, forexample integrated optic delay lines. As discussed previously, theoutputs from the second coupler 2 (incorporated within the optical phasedetector 1) vary in level depending on the relative phase of the inputsignals to the coupler 2. The optical phase detector 1 therefore providea DC output voltage which depends on the relative phase between of thetwo inputs. For example, if the delay line has a delay of T_(d) seconds,the phase shift will vary by 2π radians for every 1/T_(d) change in theinput laser signal frequency. The frequency discriminator 60 thereforeoperates with a control characteristic which repeats every 1/T_(d)change in frequency. As shown in FIG. 3 the discriminator output voltage20 (i.e. the output voltage from the differential amplifier 6) variessinusoidally with the relative optical phase of the two input signals.

If the optical phase detector is arranged to operate around 0V, abipolar output signal is produced, as described previously. If the inputfrequency to the first coupler varies in time, this will result in achange in the bipolar output signal. For an increased laser outputfrequency, the bipolar output signal from the discriminator will be ofone polarity and for a decreased laser output frequency the signal fromthe discriminator will be of the opposite polarity. The magnitude of thebipolar output depends on the degree of phase shift and hence thefrequency shift of the input laser. Typically, the peak to peak voltagemay be in the range between +/−0.1-10 V.

A frequency discriminator has useful applications itself, for example tomeasure the output spectrum of a laser. The electrical output from thefrequency discriminator 60 may be used to provide a measure of theoptical frequency (or wavelength) of an input laser as long as therelative delay between fibres 50 and 51 is known and is substantiallystable. This does provide a frequency (or wavelength) measurement,although it is an ambiguous one as the phase is only measured modulo 2π.A further ambiguity arises from the sinusoidal response shown in FIG. 3,as the maximum unambiguous phase range is π. This is, however, a usefulmeasurement over a small frequency range. The delay line length may beadjusted to set the required sensitivity. Ideally, the phase excursionwill be kept to within a few degrees around zero to maintain linearity.Alternatively, and preferably, the linearised optical phase detector 30(as shown in FIG. 5) may be employed within the discriminator 60 tomaintain linearity and range.

The bipolar output signal of the frequency discriminator may also beexploited as a means of correcting for any phase deviation in the laseroutput 42. Referring again to FIG. 8, this correction process may beachieved by means of a laser stabilisation apparatus (within outerdashed line and referred to generally as 70). The laser stabilisationapparatus 70 comprises the frequency discriminator 60 and a feedbackcircuit. The laser output 42 to be stabilised is split into two signals,preferably of equal amplitude, by the coupler 41. Preferably, the output42 from the laser 43 is passed through an optical isolator 62 to removethe detrimental effects of any light reflected back into the laser 43.The output 20 from the discriminator 60 is then fed back to the laser43, so as to vary its frequency, through the feedback circuit. It isimportant in the laser stabilisation apparatus 70 that the discriminatoroutput 20 is a bipolar output so that a frequency shift in eitherdirection can be corrected to stabilise the laser output 42.

The feedback circuit comprises a control loop amplifier 72 and a loopfilter 74. The output 20 from the discriminator 60 is amplified by thecontrol loop amplifier 72 and is then passed through the loop filter 74to produce an error signal. This error signal may then be used tocontrol the frequency of the laser 43. An error signal of the correctsign is applied to the frequency control point so as to reduce frequencyfluctuations and, hence, improve the phase noise spectrum. The“stabilised” output 76 from the laser may be taken from the apparatus byusing an additional coupler 78 located in the path before thediscriminator 60.

In the case of a semiconductor laser, such as a laser diode, thefeedback may be applied by varying the laser current, as the laserfrequency varies with current, the dominant mechanism being the changein refractive index, due to changes in the effective refractive indexwith current injection. Alternatively, it is be possible to build into alaser cavity a reverse-biased section such that it does not absorb lightbut changes refractive index with an applied voltage which derives fromthe discriminator output.

The function of the laser stabilisation apparatus is to reduce theoutput of the differential amplifier to substantially zero, thusmaintaining the laser frequency at one of the stable operating points ofthe frequency discriminator. The laser will eventually lock to thenearest stable operating point and, once lock is achieved, the systemmaintains control of the laser frequency at that particular frequency.The system may therefore be used to improve the spectral stability ofthe laser.

Measurements have been obtained for a Lightwave Electronics Series 123Fibre-coupled Diode pumped solid state non-planar ring laser controlledby a Series 2000 LNC Laser and Locking Accessory (LOLA). The laser had afrequency control input with a tuning range in excess of 30 MHz and abandwidth of 100 kHz for small modulation indices and had a quotedlinewidth of 5 kHz. FIG. 9(a) shows the low frequency spectrum measuredat the output from the discriminator and shows the phase noise (dBc/Hz)as a function of offset frequency in free running conditions (o) (i.e.measuring the laser output directly) compared to the phase noisemeasurements obtained when using the laser stabilisation apparatus 70.The measurements show a considerable improvement in the close-in phasenoise performance for a laser which already has a quoted linewidth ofaround 5 kHz (i.e. spectrally pure). In these measurements thedifferential delay was 1 μs. Additionally, FIG. 9(b) is a similarspectrum obtained for an E-Tek DFB laser, comparing the phase noise(dBc/Hz) as a function of offset frequency in free running conditions(i.e. open loop, measuring the laser output directly) compared to thephase noise measurements obtained when using the laser stabilisationapparatus 70. In these measurements, the differential delay was 5 ns.

As lasers with high spectral purity tend to be relatively expensive, thesystem enables less expensive and spectrally less pure lasers to beimproved relatively cheaply and easily. The improvement of low costsemiconductor lasers may therefore also be achieved using this system.

The most common, cheap and widely used laser is a laser diode but it hasa particularly poor spectral stability, especially in the case ofFabry-Perot etalon designs. The laser stabilisation apparatus may betherefore be particularly useful for improving the spectral stability ofthese devices, as well as DFB lasers. It may also be used to improvestable lasers which is also very useful, for example in high resolutionspectroscopy and in frequency standards. In particular, the degree ofphase noise improvement which may be obtained is dictated by the choiceof the delay time. Typically, the delay line may introduce a relativedelay of up to 10 μs between the output signals from the first coupler,depending on the range and sensitivity needed to give the requireddegree of spectral improvement. The delay time must also be chosen sothat the open loop gain of the discriminator is reduced to well belowunity at frequencies approaching the reciprocal of the delay time. Forexample, for a delay line of 1 μs, the open loop gain of thediscriminator should be well below unity for input frequencies of around1 MHz. If lasers, such as laser diodes, have a high noise level atgreater offset frequencies a wideband loop (and short delay) must beused. Thus, for lower performance lasers, much higher loop bandwidthsare required. A lower discriminator gain may be compensated for byincreasing the gain of the control loop amplifier. Johnson noise fromthe amplifiers is likely to be the limiting factor in this gainincrease.

Usually, laser diodes have two outputs, one from each facet, or mirror,the second output usually being used to monitor power level. However, itmay be possible to use the second output as the input to one of the twooptical detectors, hence alleviating the need for the first coupler.However, this embodiment may only be used if the two outputs from thelaser are strongly correlated.

In an alternative embodiment of the invention, the laser stabilisationapparatus may comprise two or more loops in parallel. For example, asecond output may be taken from the laser 43, or a split signal fromoutput 42, and fed through a second loop having a different gainbandwidth from the first feedback loop. Each loop may then be used tofeed back and control the input laser. For example, different feedbacksignals from each loop may be used to control separate temperature andfrequency controls points on the laser, or may be combined and fed backto the same control point on the laser. Alternatively, the output fromthe discriminator 60 may be split, with one split signal feeding back tothe laser 43 through one control loop amplifier and loop filterarrangement, and the other signal feeding back to a different controlport on the laser via a second control loop amplifier and loop filter.

The laser stabilisation apparatus of the present invention isadvantageous as it is an in-fibre system and is therefore is lesssusceptible to external factors, such as vibration, temperature changesand dust, than a free space system. This is especially true iftemperature-stable optical fibre is used, as discussed previously. Theapparatus may also have a higher effective Q-value compared to thatwhich may be obtained with the known device, the Q-value depending onthe length of fibre and therefore, indirectly, on the original laserstability. Furthermore, the length of the fibre optic delay line, andhence the relative delay may be selected in order to vary thesensitivity and range depending on the degree of spectral improvementrequired. The apparatus may be particularly useful for stabilising theoutput from laser diodes and other forms of laser, for example to allowthem to be used more effectively in fibre optic communications systems.

Referring to FIG. 10, it is sometimes desirable to frequency modulate alaser e.g. to transmit information, or slowly sweep the frequency of thestabilised laser. This may be done by introducing an electrooptic phasemodulator 80 or a differential amplifier arrangement 82, 83, as will bedescribed hereinafter. For example, an electro-optic phase modulator 80may be included in the laser stabilisation apparatus 70, for example ineither of the delay lines 50, 51 (delay line 50 in FIG. 10).Alternatively, the electro-optic phase modulator 80 may form part of theoptical phase detector itself, as described hereinbefore. In use, thelaser frequency may be modulated by first stabilising the laser 43 sothat the output from the differential amplifier 6 of the optical phasedetector 1 is maintained at, say, zero volts and then by applying anappropriate voltage waveform to the electro-optic phase modulator 80.The linearity of the phaseshift in the electro-optic phase modulatorensures a high fidelity frequency-modulated output.

An application of this technique may be to sweep the input laserfrequency slowly through an atomic absorption line to make precisemeasurements of the lineshape. This may be especially useful in themeasurement of very narrow lines due to the spectral stability of thelaser.

The apparatus of FIG. 10 may also be used to convert the stabilisedlaser output 76 into a stabilised optical synthesizer. In thisapplication, the voltage waveform applied to the phase modulator 80 mayhave the form of a “SAWTOOTH” voltage waveform, in which the phase isswept slowly and linearly over 360° and then rapidly switched backthrough 360°. However, other waveforms may also be used for thisapplication. Suitable waveforms include those which slowly increase withtime and then rapidly decrease to the initial voltage level. Theincreasing level does not therefore have to vary linearly with time, asin the case of a SAWTOOTH waveform. For the purpose of thisspecification, any waveform having a slowly increasing voltage levelwith time followed by a rapid switch back through 360° (in the oppositedirection) shall be referred to as a SAWTOOTH-like waveform. Suchwaveforms include waveforms in which the slowly increasing voltage levelis of stepped form. The SAWTOOTH-voltage waveform may or may not beswept linearly over 360°. Typically, the timescale over which thewaveform is switched back through 360° will be of the order ofnanoseconds.

By way of example FIG. 11(a) shows an example of a SAWTOOTH voltagewaveform (as a function of time) which may be applied to theelectro-optic phase modulator 80 for this purpose and FIG. 11(b) showsthe corresponding change in phase of the modulator 80 with time. Duringthe switching process the laser frequency first follows the slow changein voltage waveform (i.e. modulator pulse) but does not follow the rapidreduction in phase by 360° because the feedback loop filter (i.e. 72,74) will not respond at this speed. In other words, the switch through360° degrees has no net effect because the laser and feedback loop areunable to respond to a rapid change. In addition, the output of theoptical phase detector 1 is unaffected by the change of phase of 360° asits response is periodic with 360° (as shown in FIG. 3). The laseroutput 76 therefore remains at the new frequency. This process may berepeated many times to cover the full tuning range of the laser. Achange of frequency in the opposite direction can be achieved byreversing the voltage waveform slope.

Using the apparatus shown in FIG. 10, the stabilised laser output 76 canbe controlled to any required frequency by applying an appropriatevoltage 86 to the phase modulator 80. In some circumstances this isuseful, for example it allows the response over a small frequency rangeto be measured with a continuous linear sweep rather than as a series offrequency intervals.

As an alternative to an electro-optic phase modulator, a differentialamplifier may be included in the feedback loop to provide a similarlaser-tuning function. For example, referring to FIG. 10, a differentialamplifier 82 may be included in the signal pathway at X. Alternatively,the differential amplifier may form part of the control loop amplifier72 itself or may be placed in the path prior to the control loopamplifier 72. In either of these arrangements, rather than maintainingthe output 20 from the discriminator 60 at 0V, the input 83 to thedifferential amplifier 82 may be set to a constant value say, 1 V. Thedifferential amplifier 82 in the feedback loop measures the differentialbetween 1 V and the output voltage from the discriminator 60 afterpassing through the control loop amplifier 72 and loop filter 74. Thedifferential output 84 is then fed back to control the input laser 43whose frequency changes to make the output of the loop filter approachIV. This would also allow a frequency modulation capability. By thismeans the laser may be swept in frequency enabling, for example,measurement of narrow spectral line widths and shapes.

It will be appreciated that, if the differential amplifier forms part ofthe control loop amplifier 72 itself or is placed in the path prior tothe control loop amplifier 72, the operating characteristics will bedifferent from an arrangement in which the differential amplifier islocated at X, as the amplifier is located on a different side of thefilter in each case. The use of the differential amplifier 82 instead ofthe electro-optic phase modulator 80 is a cheaper approach and may beimproved by using a linearised optical phase detector (as shown in FIG.5) in place of an optical phase detector having a nonlinear response (asshown in FIGS. 1, 8 and 10). The use of the electro-optic phasemodulator 80 is the preferred approach, however, and has been built andtested in the laboratory.

FIG. 12 shows the results obtained using the optical synthesizer shownin FIG. 10, including the electro-optic modulator 80. The figurecomprises eight spectra measured on an electrical spectrum analyser bymixing the optical synthesizer output at 76 with the output of a stablefixed laser frequency. In this demonstration the discriminatordifferential delay is 1 microsecond, so successive laser frequencies are1 MHz apart. However, to provide an intelligible plot the spectra weretaken every ten “cycles” of the phase-modulator (i.e. every ten SAWTOOTHwaveform cycles and 10 MHz intervals). The scales on the plot shown inFIG. 11 are single sideband phase noise dBc/Hz at 10 dB/division(vertically) and 10 MHz/division (horizontally).

Although FIG. 12 shows every 10^(th) member of a “comb” of frequenciesspaced by 1 MHz, it will be appreciated that the synthesizer may betuned to a frequency between adjacent comb frequencies by applying theappropriate voltage to the modulator 80. For example, 16 steps may beprovided by using a computer processor and a 4-bit digital to analogueconverter (DAC), whereby the DAC converts digital controls from thecomputer processor into an analogue voltage to be applied to theelectro-optic phase modulator 80. This would introduce phase steps of22.5°, the 16^(th) step being 360° corresponding to a 1 MHz step in FIG.12.

The optical frequency synthesizer may also be employed in combinationwith a diffractive element, such as a zone plate lens or a diffractiongrating, to provide a programmable function. For example, the outputfrom the optical frequency synthesizer may be used to illuminate a phasereversal zone plate lens, the focus of which is dependent on wavelength.By varying the voltage applied to the electro-optic phase modulator 80forming part of the optical frequency synthesizer, the wavelength of theoutput from the synthesizer can be varied and, thus, the focus of thelens can be varied. This may be used, for example, to access differentlayers of an optical memory, such as a compact disc.

Alternatively, the optical frequency synthesizer may be used toilluminate a diffraction grating to “steer” the laser output. This maybe used, for example, as a means of accessing a hologram.

FIG. 13 shows how the linearised optical phase detector 30 of thepresent invention (as shown in FIG. 4) may be used in sensorapplications where the measurand modifies the properties of opticalfibre 50 (or 51). For example, the sensor may be used to measure any oneof temperature, pressure, strain, displacement, vibration, magneticfields or magnetic field gradients, electric current, electric field,voltage, chemical species, biochemical parameters, medical parameters orcommunications characteristics. For example, referring to FIG. 13, ifthe output 20 from the differential amplifier 6 is set to zero volts byvarying the voltage 36 applied to the electro-optic phase modulator 35,any variation in temperature, (or other measurand) of the optical fibredelay line 50 will give rise to a change of phase, hence the output fromthe phase detector will vary. The amount by which the voltage 36 on theelectro-optic modulator 35 varies to maintain the output 20 from thedifferential amplifier 6 at zero (or very close to zero) provides anindication of the variation in temperature (or other measurand). Theoutput of such a measurement will typically be very linear.

Although it may be advantageous to use temperature stable optic fibrethroughout the sensor, for a temperature sensor application the use oftemperature stable optical fibre for the delay line 50 is notappropriate. In such schemes a stable input laser 43 should be used. Astabilised laser input may be achieved by using the laser stabilisationapparatus of the present invention, as described previously.

In the sensor application, as distinct from the frequency discriminatorapplication, the two optical fibres 50, 51 may have substantially thesame delay, rather than having a relative delay therebetween. In somecircumstances this may be of benefit. For example, if measuring thepressure on fibre 50, any temperature variation will be common to bothfibres 50 and 51 and will therefore cancel if the delays are equal.Similarly, the system can employ a less stable laser. It is alsopossible to implement the sensor as a free-space arrangement, ratherthan an in-fibre apparatus, as will be described later.

The optical frequency synthesizer of the present invention may also beemployed in an improved form of distributed fibre optic sensor.Conventional fibre optic sensors employ an array of Bragg gratingsdistributed along the length of the optical fibre. The individualgratings can then be interrogated by means of a tuneable laser, tunedclose to the Bragg frequencies. By employing the optical frequencysynthesizer of the present invention in a distributed fibre optic sensorarrangement, advantages are obtained due to the improved stability ofthe laser and, in addition, because the laser frequency can be set withhigh accuracy. As a consequence, it would be possible to implement sucha sensor with an increased number of Bragg gratings, permitting agreater number of interrogation points to be used. An additionaladvantage of this fine-tuning capability is that it enables the overalldistortion of a grating to be sensed at narrow frequency intervals. Forexample, the reflection bandwidth of a periodic grating will increase ifdifferent paths are affected differently by the measurand.

Referring to FIG. 14, the linearised optical phase detector (as shown inFIG. 5) may also be employed in an optical vector voltmeter 90 whichoperates at optical frequencies. The optical vector voltmeter comprisesa linearised optical phase detector 30 (enclosed within the dashed line)which has an operating range of at least 360°. The function of thevector voltmeter is to take two inputs 92, 94 and to generate twooutputs, one of which (output 100) is the absolute amplitude of theunknown input and the other of which (output 102) is the phase of theunknown input signal 92 relative to that of the reference input 94. Theunknown input signal 92 is split and one part is input to a detector 96to provide the amplitude output 100. Typically, the detector currentvaries linearly with optical intensity, from which the amplitude isreadily derived. The second part 93 of the split input signal is inputto the linearised optical phase detector 30, along with a referenceinput signal 94. The output from the differential amplifier 6 is fedback to the electro-optic phase modulator 35 via a feedback amplifierand filter 44, 46. The voltage applied to the electro-optic phasemodulator 35 is adjusted, via the feedback loop, to maintain thedifferential amplifier 6 at a constant value, very close to zero. Theelectro-optic phase modulator input voltage provides the phase output102. This gives a direct linear measure of the relative phase betweenthe unknown input 92 and reference input 94.

FIG. 15 shows a schematic diagram to illustrate how the optical vectorvoltmeter 90 shown in FIG. 14 and the apparatus shown in FIG. 10 may beemployed in an optical network analyser for transmission measurement ofa system under test (e.g. an optical fibre of unknown properties). Theoptical network analyser is similar to the optical vector voltmeter,except that it provides a synthesised frequency input to the systemunder test 110 and typically may measure the transmitted amplitude andphase of the system 110 at a series of frequencies as the opticalfrequency synthesizer 108 is stepped and/or swept in frequency. As inthe optical vector voltmeter, the linearised optical phase detector isarranged to have an operating range of at least 360°. The opticalnetwork analyser of the present invention is not limited to including anoptical frequency synthesizer as herein described, and may include anyoptical frequency synthesizer apparatus.

Referring to FIG. 15, the optical network analyser comprises an opticalsynthesizer 108 whose output is split. The operation of the opticalsynthesizer is described above with reference to FIG. 10. The stabilisedlaser output 76 shown in FIG. 10 provides the optical synthesizer outputto be split. One part of the split signal 76 a is transmitted to thesystem under test 110 and is transmitted 92 to an optical vectorvoltmeter 90 (input SYS) as described above (i.e. input signal 92 inFIG. 14 is transmitted to the optical vector voltmeter 90). The otherpart of the split signal 76 b is input to the reference port (input REF)of the optical vector voltmeter 90 (i.e. reference signal 94 in FIG.14). The outputs 100, 102 from the optical vector voltmeter 90 form thenetwork analyser outputs. Typically, measurements may be made over arange of optical frequencies (by sweeping and/or stepping the opticalsynthesizer, as described previously). Other properties, such as groupdelay, can be calculated digitally using the phase and frequencymeasurements. Results may be displayed on a VDU, or in digital form.

A variety of other measurements are also possible. For example theamplitude and phase of signals reflected by a system under test 110 maybe measured using the optical network analyser shown in FIG. 16 which isessentially a rearrangement of the optical network analyser shown inFIG. 15. In this embodiment, the input signal 76 is transmitted (via adirectional coupler 112) to the system under test 110 and provides areference signal (REF) to the optical vector voltmeter 90. However, inthis embodiment, the signal 114 reflected back from the system 110provides the other input to the optical vector voltmeter 90 via thecoupler 112. In practice, the system will also be terminated by a load,T.

It may be preferable to use temperature stable fibres throughout theapparatus of the present invention (e.g. the laser stabilisation system,the optical synthesizer, the frequency discriminator, the optical vectorvoltmeter, the optical network analyser, the optical phase detector orin sensor applications), both for connecting optical fibres usedthroughout the apparatus as well as the one or more delay lines 50, 51.Temperature stable fibre is commercially available. The use oftemperature stable fibres enables the frequency of the laser output tobe stabilised to a higher degree as changes in ambient temperature willhave a reduced effect on the optical delay paths. Alternatively, thefibres may be placed in a temperature controlled environment, or theoptical path length may include an auxiliary temperature dependent pathin series with the fibre in order to maintain a constant delay period.Alternatively, or in addition for extra stability, the apparatus may beoperated in a temperature stable oven. Polarisation maintaining opticalfibre, temperature stable single mode optical fibre or temperaturestable polarisation maintaining optical fibre may also be used.

Although most optical measurements of the kind herein described areinvariably performed using fibre optics, the medium used in fibre opticcommunications, there are some circumstances in which applications ofthe kind described may be performed using free space optics. Theapparatus shown in the various aspects of the present invention can beimplemented in this way. For example, in gas sensor applications thepresence of a gas species in air can be detected through dispersionassociated with an absorption line since the dispersion causes an actualphase shift. Referring to FIG. 17(a), the apparatus shown in FIG. 13 isalternatively implemented by employing laser beams in free space (asopposed to in-fibre) by collimating and refocusing the radiation withlenses.

The sensor shown in FIG. 17 comprises an input coupler 178, or beamsplitter, for receiving a beam of laser radiation 76 and providing twooutput beams of radiation, lenses 180, 182 to perform beam collimationand lenses 190, 192 to refocus the beams of radiation into the inputpaths 3, 4 of the phase detector shown in FIG. 5. As discussedpreviously, in this application the two free space paths 194 and 196(corresponding to paths 50 and 51 in FIG. 13) may be of substantiallyequal length, one containing pure air (path 196) and one beingcontaminated by gas (path 194). Typically, the paths 194, 196 may behollow tubes.

In an alternative embodiment, with reference to FIG. 10, the opticalphase detector forming part of the discriminator in the laserstabilisation apparatus may include a second electro-optic phasemodulator in the path of the input 4. In this case, a DC voltage may beapplied to the first electro-optic phase modulator 35 to set thefrequency of the laser, and an RE voltage may be applied to the secondelectro-optic phase modulator. This provides a convenient way ofapplying RF modulation to the laser frequency. Other benefits may alsobe obtained by including a second electro-optic phase modulator in theapparatus. The optical phase detector in FIGS. 4 and 5 may also includea second electro-optic phase modulator in the path of the input signal4. In this way, for example, the “tuning” range may be extended, andeven doubled, by arranging the electro-optic phase modulators in such away that they both contribute to the tuning range.

It will be appreciated that, for the purpose of this specification, thephrase “optical” is not intended to be limited to visible wavelengths,and includes, for example, infra red wavelengths and ultra violetwavelengths.

1. A laser stabilisation apparatus for stabilising the output frequencyfrom a laser source of radiation, the laser stabilisation apparatuscomprising, a frequency discriminator apparatus comprising input meansfor receiving a primary optical input from the laser and for producingtwo primary optical outputs, means for introducing a relative delaybetween the two primary optical outputs, an optical phase detector,wherein the optical phase detector comprises means for receiving the twoprimary optical outputs and producing two combined optical outputs,detection means for detecting the intensity of each of the two combinedoptical outputs and converting the intensity of each of the combinedoptical outputs into an electrical signal, and means for measuring thedifference between the two electrical signals and generating an outputdifference signal, feedback means for feeding back the output differencesignal from the optical phase detector of the frequency discriminator tothe laser.
 2. The laser stabilisation apparatus of claim 1 comprising atleast one additional frequency discriminator apparatus, each frequencydiscriminator apparatus having corresponding feedback means for feedingback the electrical output from the associated optical phase detector tothe laser.
 3. The laser stabilisation apparatus of claim 2, wherein theoutputs from the optical phase detectors of the different frequencydiscriminators feed back to different control points on the laser. 4.The laser stabilisation apparatus of any of claims 1-3, wherein theoptical phase detector includes a voltage-controlled electro-optic phasemodulator for modulating the phase of the of the primary optical outputswhich is input to the optical phase detector, the electro-optic phasemodulator having a substantially linear response.
 5. The laserstabilisation apparatus of any of claims 1-3, including a differentialamplifier, the output from the optical phase detector being fed back toan input of the differential amplifier, the output from the differentialamplifier being fed back to the laser.
 6. The laser stabilisationapparatus of any of claims 1-3, wherein the optical phase detectorforming part of the laser stabilisation apparatus comprises couplingmeans for receiving the two optical inputs and producing the twocombined optical outputs.
 7. An optical frequency synthesizercomprising: the laser stabilisation apparatus of claim 1 for stabilisingan output from a laser, and means for varying the frequency of the laseroutput.
 8. The optical frequency synthesiser of claim 7, including twooptical fibres for introducing a relative delay between the two primaryoptical outputs, the two optical fibres having different optical pathlengths.
 9. The optical frequency synthesiser of claim 8, comprising anelectro-optic phase modulator arranged in the path of one of the lengthsof optical fibres, whereby application of a SAWTOOTH-like voltagewaveform to the electro-optic phase modulator gives rise to a variationof the frequency of the laser output.
 10. The optical frequencysynthesizer of claim 9 and further comprising a voltage source,providing a SAWTOOTH-like voltage waveform, for applying a voltage tothe electro-optic phase modulator.
 11. The optical frequency synthesiserof claim 7, comprising a differential amplifier, the output from theoptical phase detector being fed back to an input of the differentialamplifier, the output from the differential amplifier being fed back tothe laser.
 12. The optical frequency synthesiser of claim 7, wherein theoptical phase detector includes an electro-optic modulator.
 13. A methodof stabilising the output frequency from a laser comprising the stepsof; providing a frequency discriminator apparatus comprising inputmeans, inputting a primary optical input from the laser to the inputcoupling means and producing two primary optical outputs, introducing arelative delay between the two primary optical outputs, inputting thetwo primary optical outputs to an optical phase detector, comprisingcoupling means for receiving the two optical inputs and producing twocombined optical outputs, detecting the intensity of each of the twocombined optical outputs; converting the intensity of each of thecombined optical outputs into an electrical signal, measuring thedifference between the two electrical signals and generating an outputdifference signal, and feeding back the output difference signal fromthe optical phase detector of the frequency discriminator to the laser.